Also tweak the function for making groups of entity so that
it only adds model scales for vehicles, and so that the directory
it works in can be set. This still needs to be spun out into a more
generic function, though.

Also switches from setting an invalid attribute to using
the attribute dataset to store the old unit for the unit
selectors and defers loading a scene until everything
is configured.

Firefox won't draw SVGs to a canvas correctly without width/height
attributes (even if a viewBox is present). This commit inserts those
values where needed and removes the Firefox check from the click
detection code, so it works properly (and screenshots are fixed, too).

Entities positioned above the ground were going off the screen.